The following kit companies include early keyboards among their available kits:

Lark in the Morning
PO Box 1176
Fort Bragg, California 95437
707-964-5569 voice
707-964-1979 fax
email: larkinam@larkinam.com
Web site: http://larkinam.com/
This company offers an incredible array of other instruments, from the ridiculous to the sublime, but all difficult to find.

Musicmaker's Kits
PO Box 2117
Stillwater, Minnesota 55082

There is also a listserve specifically for those interested in harpsichords and other early keyboard instruments. Send email to listserve@albany.edu with the
following in the body:
subscribe hpschd-l yourfirstname yourlastname
(Note: that's an ell, not a numeral one]